1. Field of the Invention
This invention relates to an ultrafiltration process for recovering solvent from a foots oil solution. More particularly, this invention relates to a process comprising warming up cold slack wax and mixing it with solvent to dissolve the foots oil and recovering solvent from the foots oil solution by contacting the solution with one side of a semi-permeable membrane made from regenerated cellulose which permeates through at least a portion of the solvent from the foots oil solution.
2. Description of the Prior Art
Warm-up deoiling of lube oil slack wax is wellknown in the art. In such a process the slack wax is warmed up and mixed with solvent to dissolve the foots oil in the slack wax, thereby forming a slurry comprising solid particles of wax and a solution of foots oil and solvent. This slurry is then filtered, usually in a rotary drum filter, to separate the foots oil solution from the solid wax particles as a filtrate. The foots oil-containing filtrate is then sent to solvent recovery operations to recover the solvent from the foots oil solution, with the recovered solvent then recycled back into the warm-up deoiling operation. The prior art solvent recovery is accomplished by thermal means such as passing the solvent-containing oil through a distillation tower or a series of distillation towers and/or evaporation zones in order to boil off the solvent from the oil. Residual amounts of solvent and oil are generally removed therefrom via steam or inert gas stripping. These solvent recovery operations require considerable amounts of thermal energy, pumps, tankage, etc., in order to remove the solvent from the foots oil. It would be an improvement to the art if a warm-up deoiling process could be developed that included a method for separating solvent from the foots oil by relatively low-energy consuming non-thermal means.
It is also well-known in the art to use semipermeable membranes for hydrocarbon separtion processes. Such processes are often referred to as reverse osmosis or ultrafiltration processes. In such processes, the feed, comprising a mixture of at least two different hydrocarbons, is brought into contact with one side of a membrane across which a pressure differential exists, for a period of time sufficient to form two solutions; a permeate which passes through the membrane and a retentate. A useful review of these processes may be found, for example, in an article titled, "Novel Device and Process-Design Concepts For a Large-Scale Membrane Separation of Hydrocarbon Mixtures," by Michaels et al, Seventh World Petroleum Congress Proceedings, v. 4, Page 21 (1967). It is also well-known to use membranes such as various cellulose esters including cellulose acetate, cellulose butyrate, cellulose propionate, etc., as well as cellulose ethers such as ethyl, propyl and amyl cellulose and the like for these processes. U.S. Pat. No. 2,930,754 discloses the use of cellulose ester membranes such as cellulose acetate-butyrate for separating various hydrocarbons such as n-heptane from mixtures of n-heptane and isooctane and toluene from mixtures of toluene and isooctane, using membranes at pressure differentials ranging from about 10-100 psig across the membrane. Where the feed is a mixture of toluene and isooctane, for example, the permeate is richer in toluene while the retentate is richer in isooctane. Using a series of membranes results in substantial separation of one hydrocarbon from the other. U.S. Pat. No. 2,958,657 discloses the use of membranes made of ethyl cellulose for separating hydrocarbons such as n-heptane and isooctane wherein the temperature of the process is sufficiently hot to cause one of the hydrocarbons (in this case isooctane) to vaporize and form an isooctane-rich permeate. That is, the hydrocarbon mixture is present in the feed zone in the liquid phase whereas the permeated hydrocarbons are removed as vapors. U.S. Pat. No. 2,985,588 discloses hydrocarbon separation wherein the rate of permeability across or through the membrane is increased by adjusting the temperature of the process so that it is above the first order transition temperature of the polymeric material comprising the membrane. Membrane materials disclosed in this patent include cellulose triacetate, ethyl cellulose and irradiated polyethylene. U.S. Pat. No. 2,960,462 discloses the use of dual layer membranes for hydrocarbon separation wherein the membrane is a composite of a higher permeation material and a lower permeation material such as cellulose esters and cellulose ethers or cellulose esters and irradiated polyethylene, etc. U.S. Pat. No. 2,958,656 discloses that the rate of selective permeation of hydrocarbons through non-porous cellulose ester and acetate membranes can be increased many fold by contacting the membrane, during permeation, with a non-hydrocarbon solvent material including oxygenated compounds such as alcohols, ethers, alcohol ethers, ketones and chlorinated compounds. Similarly, U.S. Pat. No. 2,947,687 discloses that the permeation rate through a non-porous, semi-permeable cellulose ester membrane may be improved by contacting the membrane, during the permeation process, with a substituted hydrocarbon which is soluble in and has solvent power for the membrane. Still further, U.S. Pat. No. 3,043,891 discloses a similar process and achieves a similar purpose by using aromatic and unsaturated nonoxygenated solvents. Unfortunately, adding a membrane solvent to the feed causes the membrane to soften and become very weak which often results in membrane rupture. Thus, U.S. Pat. No. 2,923,749 discloses adding saturated hydrocarbons to the feed mixture in order to act as a diluent thereby permitting increased permeation through the membrane at a lower temperature or pressure at the expense of membrane solubility. It is also important to note that all of these prior art processes maintain a vacuum on the permeate side of the membrane so that the permeate is removed as a vapor.
Thus, it has generally been concluded that cellulose acetate membranes are not, in general, suitable for organic feed mixtures, even though they appear good enough for other (i.e., aqueous separations) applications (c.f., Reverse Osmosis by Sourirajan, Chapter 7, Academic press, 1970). In Membrane Processes for Effluent Treatment by D. Pepper, Chemistry and Industry, Pages 834-836 (Oct. 15, 1977), it is stated that commercially used reverse osmosis membranes are usually made from cellulose acetate or polyamide. Finally, the French have used acrylonitrile copolymer membranes in a reverse osmosis process for the ultrafiltration removal of impurities from used motor oils as disclosed in an article titled, "Regeneration of Used Lubricating Oils by Ultra-Filtration," by D. Defives et al, Information Chimie, No. 175, Pages 127-131 (March, 1978). This article also states that if these acrylonitrile copolymer membranes are used in the presence of a solvent immiscible with water, such as a hydrocarbon solvent or oil, they are not wetted and are thus impermeable to the hydrocarbon. In order to function in a non-aqueous medium, the article states that the membranes must be conditioned by using a solvent that is both miscible with water and with the solvent under consideration, such as acetone or a low molecular weight alcohol. However, it was found that if the oil contained a sufficient amount of surface-active agents and was under sufficient pressure, the oil succeeded in progressively wetting the moist membranes, thus eliminating the solvent conditioning required on a laboratory scale.
Interestingly, it has been stated with authority that the most useful membranes to convert for use with organic liquids are cellulose membranes, but that cellulose acetate or cellulose nitrate membranes are soluble in too large a variety of organic solvents to be useful. Progress in Separation and Purification, v. 3, Perry and VanOss, eds., Pages 105-106; 122-124 (Wiley-Interscience, 1970). This reference states that in order to impregnate a hydrophilic membrane with a water-immiscible liquid, the membrane has to be soaked in a series of successive baths of the following composition: 30% water-70% ethanol, 5% water-45% ethanol-50% butanol, 100% butanol (renewed 3 times), 70% butanol-30% oil, 30% butanol-70% oil, 100% oil. Cellophane as such could not be impregnated with any non-aqueous liquid that was tried unless it was first treated with concentrated ZnCl.sub.2. Unfortunately, this treatment destroys the membrane selectivity required for many hydrocarbon separations.
Therefore, because regenerated cellulose or cellophane membranes are not soluble in solvents uses for dewaxing hydrocarbon oils such as lube oils, are relatively low in cost, are readily available and relatively high in strength, it would be an improvement to the art if a lube oil slack wax warm-up deoiling process could incorporate the use of these membranes to separate solvent from the foots oil solution produced in such a process.